I'd run some shop-made compression-bent 2" pipes a few feet off the stock manifolds, you're going to lose low-end and mid-range torque putting 2.5" pipes on manifolds with 1 7/8" outlets. Has the same effect as basically running with open manifolds, there's no exhaust gas velocity at lower RPMs. And having the stock manifolds will restrict breathing at higher revs anyway, might as well try to get the most torque across the whole rev range.
Also IMO mandrel bends won't do diddly with that kind of setup, pipes will be too big anyway. Now if you were running headers it would be a different story.
